1. Field of the Invention
The invention relates generally to amplifying light in an optical signal wavelength range, and more particularly, to fiber amplifiers comprising an amplifying fiber and a pump source.
2. Description of the Related Art
Optical fiber amplifiers based on Stimulated Raman Scattering (SRS) effect, also known as Raman amplifiers, are attracting considerable interest for use in telecommunications. They provide a gain bandwidth that is sufficiently wide to allow simultaneous amplification of many spectral channels in wavelength division multiplexed (WDM) systems. Raman amplifiers are typically implemented in a distributed configuration or a discrete configuration.
Raman amplifiers in a distributed configuration amplify light in an optical wavelength range within the transmission fiber itself. In one such distributed configuration, light at a primary pump wavelength(s) (i.e., pump light) is coupled to one end of a transmission fiber. Light in an optical signal wavelength range, typically comprising many discrete wavelength channels (i.e., optical signals), is coupled into the other end of the same transmission fiber. Energy is gradually transferred from the pump light propagating in a reverse direction to the optical signals propagating in a forward direction, which is known as backward-pumped Raman amplification.
There are disadvantages, however, to conventional distributed amplifier configurations. As pump light propagates in a reverse direction in a transmission fiber, it is attenuated due to the passive loss in the fiber, so substantial amplification only takes place in about the last 20 km of the link (i.e. the 20 km nearest to the pump). Increasing pump power beyond a certain limit, typically about 800 mW, is not practical due to Raleigh backscattering in the amplifying section of the transmission fiber, which causes a significant noise increase in the optical signals being amplified.
Raman amplifiers in a discrete configuration amplify an optical signal within a section of specially designed amplification fiber packaged together with a pump source. Conventional discrete configurations typically employ only a few kilometers of amplifying fiber, so Raleigh backscattering is less significant than in conventional distributed configurations. However, discrete configurations also suffer from many problems. Some of the pump power is lost as the pump light propagates through the amplifying section of fiber, as described above with respect to conventional distributed configurations. A much greater amount of pump power, however, is lost as the pump light escapes at the other end of the amplifying fiber. Pump leakage, as this phenomenon is commonly described, is typically in the range of about 15 to about 85% of the total optical power at the primary pump wavelength(s), and is at least partially responsible for poor pump conversion efficiency of conventional discrete configurations.
Discrete Raman amplifiers typically require relatively high pump power, of an order of 1 W or more. A source commonly used to pump such amplifiers is known as a cascaded fiber Raman laser. In a cascaded fiber Raman laser, primary pump light (typically, from a rare-earth doped fiber laser) creates gain at a wavelength longer than the primary pump wavelength by the amount of so-called Stokes shift (about 60 nm to about 100 nm in silica based fibers) by stimulated Raman scattering (SRS). If an optical feedback is provided for light at this new wavelength, also called the first Stokes order, lasing at this new wavelength will occur. Lasing at the first Stokes order creates gain and causes lasing at the second Stokes order, at a wavelength longer than the first Stokes order by one more Stokes shift and so on. By generating a certain number of Stokes orders, the output of the rare-earth doped fiber laser is effectively translated to a much longer wavelength where it needs to be to create gain in an optical transmission signal wavelength range.
Several solutions for the aforementioned pump leakage problem in discrete Raman amplifiers pumped by a cascaded Raman laser have been suggested. In one such configuration, light at a primary pump wavelength, generated by a Yb fiber laser, circulates in a fiber ring formed with several wavelength-selective fiber couplers. The fiber ring is used to bi-directionally generate third Stokes order in respect to the primary pump wavelength. In turn, the third Stokes order light amplifies light at an optical signal wavelength, passing through the same fiber ring.
In another configuration, a cascaded Raman fiber laser and amplifying fiber are spliced together to form a fiber ring. Primary pump wavelength light as well as optical signal wavelength light are coupled in and out of the ring using wavelength-selective fiber couplers.
The aforementioned discrete amplifier configurations are not, however, free of disadvantages. Bi-directional generation of Stokes orders means that part of the pump light is co-propagating with signal, which can lead to a large amount of noise being transferred form the light at a primary pump wavelength to the light at an optical signal wavelength. Further, pump leakage can still be a problem due to light at the primary pump wavelength exiting the ring through the output coupler.
Designs that provide secondary pump wavelength generation and optical signal amplification in different parts of a fiber ring utilize a relatively large number of passive components such as isolators, couplers and filters, each contributing to optical losses, thereby decreasing the overall efficiency and noise figure of the amplifier, and substantially increasing the cost of manufacturing the amplifier.
Thus, a need exists for an optical amplifier with improved efficiency, low noise, and a relatively low manufacturing cost.
The present invention is directed at improving or overcoming one or more of the problems listed above, and other problems found within the prior art.
According to one embodiment of the present invention, an optical amplifier comprises an optical fiber ring, and a pump source emitting light at at least one primary pump wavelength, the light being optically coupled to the optical fiber ring and generating light at at least one secondary pump wavelength, wherein optical signal amplification and secondary pump wavelength lasing occur within the same section of the optical fiber ring.
According to an illustrative embodiment of the present invention, the pump source emits light at a plurality of pump wavelengths.
According to an illustrative embodiment of the present invention, the primary pump wavelength produces lasing at a secondary pump wavelength by stimulated Raman scattering effect in the nth Stokes order, n being an integer.
According to an embodiment of the present invention, the secondary pump wavelength lasing amplifies an optical signal by stimulated Raman scattering in the first Stokes order.
According to an embodiment of the present invention, the optical fiber ring includes an optical isolator that allows the light at a secondary pump wavelength to circulate in only one direction within the optical fiber ring.
According to an embodiment of the present invention, the optical fiber has a core and a cladding, the core comprising materials having stimulated Raman scattering cross-sections sufficiently high to provide amplification of light at a secondary pump wavelength and light at an optical signal wavelength, and the core having a refraction index sufficiently higher than the refractive index of the cladding to guide light at the primary pump wavelength and secondary pump wavelength and signal wavelength. Preferably, the core is. doped with at least one of GeO2 and P2O5.
According to an embodiment of the present invention, the pump source comprises one of a semiconductor laser, a rare-earth ion doped fiber laser and a Raman laser.
According to an embodiment of the present invention, the optical fiber ring includes two wavelength-selective optical coupler devices, the coupler devices coupling light at a primary pump wavelength and light at an optical signal wavelength in and out of the optical fiber ring and allowing light at a secondary pump wavelength to circulate within the optical fiber ring.
According to an embodiment of the present invention, the optical fiber ring includes two optical circulators, the optical circulators coupling light at a primary pump wavelength and light at an optical signal wavelength in and out of the optical fiber ring and allowing light at a secondary pump wavelength to circulate in only one direction within the optical fiber ring.
According to an embodiment of the present invention, the optical fiber ring preferably includes a wavelength-selective blocking filter positioned between the optical circulators, the wavelength-selective blocking filter substantially preventing lasing at an optical signal wavelength.
According to an embodiment of the present invention, the optical amplifier further comprises an optical reflector placed within the optical fiber ring such that light at a primary pump wavelength is reflected by the reflector.
According to an embodiment of the present invention, the optical amplifier further comprises a wavelength selective optical coupler device, the wavelength selective optical coupler device combining light at a primary pump wavelength and light at an optical signal wavelength in a single optical fiber.
According to an embodiment of the present invention, the optical amplifier further comprises a wavelength selective optical filter having a periodic transmission characteristic such that only light with a wavelength corresponding to the Stokes orders of the primary pump wavelength can pass through the wavelength selective optical filter.
According to an embodiment of the present invention, a portion of the light at a primary pump wavelength is allowed to exit the optical fiber ring for second order distributed Raman pumping an optical transmission fiber.
According to an embodiment of the present invention, the optical amplifier further comprises an even number of wavelength selective optical coupler devices connected in pairs by respective optical fiber links, the coupler devices designed to couple out and return to the optical fiber ring light at wavelengths corresponding to the even Stokes orders of the primary pump wavelength and to substantially transmit light at all other wavelengths.
According to an embodiment of the present invention, 1% to 80% of the light at a secondary pump wavelength is allowed to exit the optical fiber ring for backward distributed Raman pumping an optical transmission fiber.
According to an embodiment of the present invention, a method of amplifying an optical signal is provided comprising the steps of providing an optical fiber ring, emitting ligth a primary pump wavelength into the optical fiber ring, lasing light at a secondary pump wavelength that substantially circulates only in one direction within the optical fiber ring, and amplifying light at an optical signal wavelength within the optical fiber ring, wherein amplifying light at an optical signal wavelength and lasing light at a secondary pump wavelength occur within the same section of the optical fiber ring.